The present invention is related to a process for desulfurizing molten steel to a sulfur content of 5 ppm or less.
Even if molten pig iron is desulfurized, it is difficult to obtain molten steel with a sulfur content of 10 ppm or less. Generally, in order to obtain a low sulfur steel, the molten steel is desulfurized after pouring or changing the molten steel from the converter to a ladle.
A conventional method for desulfurizing molten steel includes blowing a desulfurizing flux into the molten steel by degassing equipment or injection equipment. Another method includes shooting calcium particles into the molten steel.
However, in these processes, in addition to high cost, since the flux is not sufficiently slagged, it is difficult to obtain an ultra low sulfur steel having a sulfur concentration of 5 ppm or less.
In another method, after a desulfurizing flux is added to the molten steel in the ladle, Ar gas is blown into the molten steel so as to stir the molten steel. However, when the molten steel is poured into the ladle, converter slag flows into the ladle from the converter. The composition of the converter slag may not be preferable for desulfurization or may be highly oxidized. For this reason, in this process, ultra low sulfur steel cannot be regularly obtained. Since the flow amount and the composition of the converter slag having a low desulfurizing efficiency change, it is difficult to optimally control the composition of the desulfurizing slag in the desulfurizing process. In addition to this, although the flux is slagged by the heat of the converter slag and the molten steel, the flux cannot always be slagged sufficiently by this process.
When MnS inclusions form in the steel, these inclusions are drawn by the rolling of the steel and serve as traps for hydrogen, thereby easily causing hydrogen-induced cracking. For this reason, pipe line steel which is particularly used in cold areas must be free from precipitation of MnS. One way to prevent MnS inclusions from occurring is to add Ca in the molten steel to precipitate CaS. In the case of CaS, even if the steel is rolled, CaS inclusions maintain spherical shapes. Therefore, hydrogen-induced cracking does not easily occur.
Various methods for adding Ca in the molten steel are known. These include an injection method in which a Ca alloy powder such as Ca-Si alloy is carried by a gas and blown into the molten steel through a lance, a shooting method in which Ca alloy particles are shot into the molten steel, and a Ca-wire method in which Ca alloy wires coated with iron are added to the molten steel. However, since Ca has a boiling point 1,483.degree. C. lower than the melting point of steel and an extremely high steam pressure, the Ca addition yield is very low, e.g., about 10%, and this yield varies widely. For this reason, the expensive Ca alloy is used wastefully. In addition to this, by adding Ca, hydrogen and nitrogen is increased and an undesirable decrease in the temperature occurs.